You do already have a Modder's workshop publisher licence, which was created on 19th March 2012; if you have not changed your email, you would have had an email sent to your gmail account on that date with the details.

Note that the login is case sensitive for the publisher licence - Aroundworld rather than aroundworld for your username.

If you need to change it, the launcher, on the 'My Moviestorm' menu is 'Create modders licence'. From there, you could delete your existing licence (both locally and on the server), and create a new one.